1. Field of the Invention
The present invention relates to a cooling method in which the quality of hot-rolled steel plates is controlled in the form of a in-line production process.
2. Description of the Related Art
In general, a hot-rolled steel plate is produced by working a desired material in a rolling step, a water-cooling step or other steps. While such hot-rolled steel plate is being conveyed on the production line, the temperature within the steel plate is normally lower at the edge portion than in the middle. The cooling in the water-cooling step is commonly effected from the widthwise edges of the steel plate to the intermediate portion therebetween, from its lengthwise ends to the intermediate portions therebetween, and from its top and bottom surfaces toward the thicknesswise center. Also, the behavior of the cooling water on the top surface of the steel differs from that on the bottom surface thereof, and this causes a difference between the cooling rates applied on the top and bottom surfaces. Accordingly, when each portion of the steel plate is cooled at a different cooling rate, an anisotropic internal stress is locally formed in the steel plate, thereby impairing the shape thereof.
Various proposals have heretofore been made with respect to methods of preventing the occurrence of such shape deformations.
However, where the prior-art methods are applied to a continuous production line for a steel plate, they encounter outstanding problems. The following description concerns typical problems relating to the prior art.
(1) In one method, the rate of the cooling water supplied onto the top and bottom surfaces of the steel plate is adjusted by considering the states of the two cooled surfaces.
As an example, the following method has generally been adopted. In order to equalize the cooled states of the top and bottom surfaces, a suitable ratio as between the supply rates on the top and bottom surfaces is empirically calculated and an associated water cooling means is controlled on the basis of the thus-obtained suitable ratio, thereby allowing the shape of the steel plate to be precisely controlled. However, this prior-art method fails to provide a completely satisfactory effect with respect to preventing the occurrence of defective shape formation in the production of steel plates. To overcome the disadvantages, the specification of Japanese patent unexamined publication No. 87914/1985 proposes a method of providing symmetrical water cooling in the direction of the thickness of the steel plate. Specifically, in this method, the temperatures of the top and bottom surfaces of the plate are measured before the commencement of spraying cooling water, the conditions for setting the rate at which water is sprayed onto these two surfaces being calculated through arithmetic operations, so that the temperature difference between the top and bottom surfaces of the water-cooled steel plate may be controlled within an allowable range, and, the rate at which water is sprayed onto these surfaces of the ensuing steel plate to be water cooled being corrected, on the basis of the value of the temperature difference measured upon completion of the water cooling. As compared with the prior-art methods employing such empirically obtained value for the rate at which water is to be sprayed onto the top and bottom surfaces, the method described in the Japanese patent unexamined publication No. 87914/1985 is capable of reducing the proportion of defective shape formation in the production of a sheet plate. However, this method cannot perfectly prevent the occurrence of such defective shape formation. This is because, even if there is no temperature difference between the top and bottom surfaces when water cooling is completed, if there is any temperature difference therebetween during the water cooling, stress is generated asymmetrically along the thickness of the steel plate, thus leading to the defective shape formation in the production of the steel plate.
(2) Proposals have been made with respect to method of cooling the middle portion of the steel plate more positively than the widthwise edges of the same. This is because, since a non-uniform temperature distribution is formed in the widthwise direction of the steel plate during control of the cooling of a hot-rolled steel plate, when the steel plate is cooled to an ambient temperature range, shape defects such as waves or cambers are formed on the steel plate.
In order to prevent such shape defects, the specification of Japanese patent unexamined publication No. 87914/1985 proposes a method of cutting off cooling water from the widthwise edge portions of the steel plate so that such portions will not be excessively cooled as compared with the center.
In addition, the Japanese patent unexamined publication No. 87914/1985 proposes a concrete control method on the premise that it is possible to control the rate at which cooling water is sprayed in the widthwise direction of the steel plate. Specifically, in this method, the temperature of the plate is measured before the commencement of water cooling, the conditions for setting the rate at which the water is sprayed onto these two surfaces being calculated through arithmetic operations so that the temperature difference in the widthwise direction of the steel plate may be controlled within an allowable range, thereby applying a water cooling to the ensuing hot rolled steel plate in a controlled manner on the basis of the value of the temperature measured upon completion of the water cooling.
In order to improve the prior-art techniques, the applicant of this invention has proposed the method disclosed in the specification of Japanese patent unexamined publication No. 174833/1985. This method contemplates the fact that, when the physical properties of the steel plate such as its linear expansion coefficient and specific heat are abruptly varied by Ar.sub.3 transformation during a water cooling step and thus Ar.sub.3 transformation proceeds in a varied manner in the widthwise direction of the steel plate, an internal stress or a plastic strain is generated in the steel plate, so that shape defects such as waves and cambers are formed on the steel plate when it is water cooled to ambient temperatures. In this method, the supply of water in the widthwise direction is controlled during a water cooling step so that the Ar.sub.3 transformation in the middle portion in the widthwise direction of the steel plate may proceed simultaneously with or after that which takes place in the widthwise edge portions of the same.
The method described in the specification of Japanese patent unexamined publication No. 174833/1985 is intended for controlling the widthwise spraying of cooling water so that the temperature difference of the steel plate in the widthwise direction may be controlled within an allowable range when the water cooling is completed. However, the present inventor has found that it is difficult to perfectly prevent the occurrence of shape defects such as waves and cambers in the steel plate merely by cooling the plate so that the temperature distribution may be uniform in the widthwise direction when the water cooling is completed.
The specification of Japanese patent unexamined publication No. 174833/1985 is intended for solving the above-mentioned problem, and is designed to control the rate at which cooling water is supplied in the widthwise direction so that, as described above, the Ar.sub.3 transformation in the widthwise edge portions of the steel plate may proceed simultaneously with or after that which takes place in the middle portion in the widthwise direction. However, in an actual application of this method, since there is presently no practical means for detecting the commencement and the end of the Ar.sub.3 transformation, the control steps must entirely rely on a forecasting type of calculation. Moreover, there is a further problem in that it is impossible to confirm the probability of the result obtained from such forecasting calculation being correct.
As can be seen from the foregoing, although the prior-art methods are theoretically proper, there is no, practicable means for carrying them out. Thus, none of them provides any effective solution to the aforementioned problems.